Optoelectronic devices interact with radiation and electric current. Such devices can be light-emitting devices that produce radiation as a result of an applied electric voltage/current or photovoltaic devices that produce an electric voltage/current as a result of applied radiation. Photovoltaic (PV) cells/devices typically employ a substrate or carrier (wafer, film, foil, etc.), a bottom electrode, one or more layers of PV material and a top electrode. Either the bottom electrode or the top electrode may be the anode and the opposite the cathode and vice versa. PV materials and layer structures are, broadly speaking, materials that create a voltage and current between the two electrodes when the PV material/layer structure is exposed to light.
In the past PV materials were limited to inorganic materials, for example, silicon (crystalline, poly-crystalline, amorphous), GaAs, CdTe, CIGS, or nano/meso-porous titania-based dye+liquid electrolyte cells (‘Graetzel cell’). Recently, organic materials have been used as PV materials. Such organic materials include semiconducting gels, conjugated polymers, molecules, and oligomers. Organic PV materials may also include porous films of sintered particles such as titania particles. These materials may or may not be doped to improve performance (e.g. reduce resistance to improve efficiency). Examples of such organic PV materials are described, e.g., in Brabec, Christoph J. Sariciftci, N. S. “Recent Developments in Conjugated Polymer Based Plastic Solar Cells” 2001, Chemical Monthly, V132, 421-431. One of the great advantages of organic or partly organic solar cells is that they can be made much thinner than e.g. silicon-based PV cells (few 100 nm as opposed to several micrometers).
PV cells may be optimized for solar-cell applications, i.e. applications in which typically outside sun/day-light impinges on the cell and the voltage and current output from the PV cell is optimized/maximized. FIG. 1 depicts a schematic diagram of a typical solar cell according to the prior art. The solar cell 100 generally includes a substrate 102, a bottom electrode 104 disposed on the substrate 102, and one or more active layers 106 disposed between the bottom electrode 104 and a top electrode 108. In such solar cell applications large currents have to be carried from the PV cell(s) to an outside electrical circuit or device. One of the PV cell surfaces, e.g., a top surface 109 has to be at least semi-transparent to collect this outside light but this light also has to penetrate through the electrode on this side. Thus one faces the problem of maximizing light transmission into the cell while minimizing the resistance of said electrode to efficiently (at low power loss) carry the collected current to the outside circuit or device. This is often achieved by using semi-transparent conducting material in at least one of the top and bottom electrodes 104, 108.
The substrate 102 may be transparent or opaque. In cases in which the light of e.g. a PV cell penetrates into the device through the bottom substrate 102 the top electrode 108 does not normally have to be transparent. In cases in which the bottom substrate 102 is opaque the light needs to reach the device (electronically/optically active layer(s)) through the top electrode 108. Naturally, both the top and bottom electrodes 104, 108 and the substrate 102 must be at least partly transparent for the case in which the light is desired to reach the active layer(s) 106 from both sides.
In the prior art, transparent conducting electrodes (TCEs) have typically been made using a transparent conducting oxide (TCO) such as indium-tin-oxide, ITO, or tin oxide, SnOx (with or without fluorine doping), Al-doped ZnOx, etc.). Such TCO layers have often been combined with metallic grids of additional lower resistance materials, such as e.g. screen-printed metal-particle pastes (e.g. silver-paste). For example, U.S. Pat. No. 6,472,594 to Ichinose et al describes coating metal with a conductive adhesive in order to attach the wires to and make electrical contact with an underlying TCO. Such approaches are still far from optimal as limited light transmission and residual resistances limit device efficiency and manufacturing is costly. Furthermore, such approaches are not compatible with the use of organic PV cells. Ichinose, in particular does not address applications involving organic PV cells.
It is known in the prior art that TCO materials, particularly where they act as anodes to extract and/or inject positive charge carriers, may not form good ohmic or near-ohmic contacts with organic p-type materials such as those employed in organic or partly-organic solar cells. Furthermore, organic or partly organic solar cells are often more sensitive to ‘process conditions’. For example, depositing a TCO layer (e.g. via the typical sputtering processes or even reactive sputtering processes that create UV and/or plasma conditions) can damage the organic layers such that cells may, for example have electrical shorts. Because organic solar cells tend to be much thinner than silicon-based PV cells, any damage and/or surface modification due to the TCO deposition process can, hence, be relatively much more relevant and damaging in an organic PV cell. Furthermore, TCO deposition processes typically employ vacuum-coating steps that are difficult and costly, even in a web-based roll-to-roll process.
Conductive polymer films, e.g. Pedot, Pani or polypyrrole, represent an alternative to TCO electrodes. Such polymer materials are far more suitable for roll-to-roll processing, as they can be solution processed/coated. Furthermore, such conductive polymer materials do not require sputtering or plasma processes to put them on an active layer. Unfortunately, after processing such as coating and drying, such conductive polymer films have sheet resistances significantly higher than TCOs; e.g., about 200 Ohms/square. Because of this, the resistive power loss would be far too high. Thus, pure conductive polymer transparent electrodes are unacceptable for PV approaches.
Therefore, a need exists in the art for an improved transparent conducting electrode that overcomes the above disadvantages and a corresponding method for making it.